Final answer:
The mitochondrion is not part of the endomembrane system because its structure is not derived from the ER or Golgi apparatus. It has a different origin, believed to be from a symbiotic relationship with a prokaryotic cell, distinguishing it from the interconnected components of the endomembrane system.
Step-by-step explanation:
The mitochondrion is not classified as part of the endomembrane system of eukaryotic cells. The correct answer to why this is the case is B) Its structure is not derived from the ER or Golgi. The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the endoplasmic reticulum (ER), the Golgi apparatus, as well as the plasma membrane. These components work together to modify, package, tag, and transport proteins and lipids that form the membranes. The mitochondrion, which has its own DNA and ribosomes, is believed to have originated from a symbiotic relationship between eukaryotic cells and a prokaryotic cell, which eventually became an integral part of the eukaryotic cell as an organelle. This endosymbiotic origin is distinct from the formation of the endomembrane system components, which are interconnected and share portions of their membranes.